1. Field of the Invention
The present invention relates to an on-the-fly optical interference measurement device for measuring a workpiece being machined, and more particularly, relates to a device in which consecutive satisfactory interference fringe images can be obtained over a wide interference fringe range. The present invention can be suitably applied to measurements while abrasive work such as lapping or polishing is being performed. The present invention also relates to a machining device provided with a measurement device as outlined above.
2. Description of the Related Art
In order to precisely finish a gauge block or a similar item, machining such as lapping, polishing, or the like is often performed using abrasive particles. With this machining method, a machine tool (such as a lapping machine) which serves as a reference and the workpiece are pressed against each other and moved relative to each other. As the workpiece and the working tool slide against each other, abrasive particles are introduced between the workpiece and the working tool. Such systems employing abrasive particles include both liberated abrasive systems and fixed abrasive systems. In a liberated abrasive system, a working fluid in which the fluid and the particles have been mixed is supplied between the machine tool and the workpiece. In a fixed abrasive system, the abrasive particles are embedded in the sliding surface of the machine tool. This second method of machining is suitable for precise finishing of surfaces, and is commonly applied, for example, to the machining of gauge blocks or other precision components, the machining of optical components such as lenses and mirrors, and precise finishing of components such as semiconductor wafers.
To measure the accuracy of surfaces or dimensions of an abrasively machined workpiece, a measurement device is used for optical detection of interference fringes. Well-known measurement devices include those such as the Fizeau interferometer, and measurement is usually performed by utilizing an image of the interference fringes formed in accordance with the surface contour of the workpiece. Optical interference measurement technology is described in documents such as, for example, "Absolute Measurement of Surface Contours Using Interferometers" (Nagahama, Lecture text for 16th Symposium on Optics (1991), Lecture No. 3, p. 55-58). Another publication, "Recent Advances in Optical Interference Measurement Methods" (Yatagai, Precision Machinery 51/4/1985, p. 65-72) describes a device for automatically determining the flatness as the surface contour, using image processing of interference fringes.
However, in the known systems, when a workpiece is set on a polishing machine, because the workpiece is covered by the machine tool, measurement cannot be performed while machining is in progress (on-the-fly measurement). As a result, the workpiece is usually removed from the machine and cleaned before being set on the measurement device, with measurement then being performed.
Generally, high machining precision in the micron to sub-micron range is required when lapping and polishing finishing is being used. When high precision is required, the workpiece is again machined after it has been cleaned and then measured. The repeated process of machining, cleaning and measuring the workpiece until the required precision is achieved is extremely intricate and complicated and machining costs for highly accurately machined parts, particularly optical components and gauges, tend to be exceptionally high.
It is therefore conventionally necessary to interrupt the lapping or polishing, remove the workpiece from the machine, and measure the contour, and this has served to inhibit improvements in productivity and machining accuracy. Given these conditions, there is a call for methods by which interference fringes in surface contours can be detected after the workpiece has been set in place on the machine, even while machining is in progress.
It is particularly desirable to find a way in which interference fringes can be detected, not only while machining is in progress, but also in which consecutive interference fringes can be obtained appropriately over a wide area. Using interference fringes from a narrow area, it is possible , but extremely difficult, to judge the surface contour through visual inspection. However, calculating surface accuracy using image processing and other means requires consecutive interference fringes in a sufficiently broad area.
The above describes an example of a conventional technical problem which occurs in abrasive machining. The problem described above, however, is not limited to abrasive machining. It is also not possible to detect interference fringes of the surface being machined using optical means, while machining is in progress in types of machining other than abrasive machining.